1. Field of the Invention
The invention is in the field of loose tube fiber optic cables.
2. Description of the Related Art
Fiber optic cables are becoming increasingly accepted in U.S. telecommunications due to their superior ability to transmit data as compared to copper cables. However, the glass fibers used in fiber optic cables are more sensitive to cable stresses; copper cables are more forgiving in this regard. As a result, loose tube fiber optic cables were developed; the optical fibers are placed in buffer tubes with room for the fibers to move within the buffer tubes. This construction decouples the glass fiber from forces applied to the cable within a given range of cable compression or elongation. Thus, the cable is able to maintain an acceptable level of performance during specified ranges of temperature and during cable bending and pulling within certain tolerances. The buffer tubes may be filled with a jelly to lubricate the fibers and retard moisture entry, further improving cable performance. The buffer tubes may contract or elongate without immediately affecting the fibers therein.
One way to construct such a loose tube fiber optic cable is to strand, or wind the buffer tubes containing fibers around a central strength member. Quite often, the central member is a circular cylinder. Also, the buffer tubes often have cylindrical shapes. When cylindrical buffer tubes are stranded around a cylindrical central member, the buffer tubes form a curve called a helix. The equation of the length L of the section of a helix over one period (0.fwdarw..theta..fwdarw.2.pi.) may be expressed as L.sup.2 =.pi..sup.2 D.sup.2 +P.sup.2, where D is the diameter of the helix and P, herein called the pitch, is the distance between adjacent loops.
Since the buffer tubes assume the shape of a helix in this construction, optical fibers contained within the buffer tubes must also follow a helical path. It will be recalled that the inner diameter of the buffer tube is larger than the outer diameter of the optical fiber, allowing the fiber relative movement within the buffer tube. The objective of the loose tube is to allow the fiber not to undergo stress, due to transverse forces from the buffer tube or tension directly on the fiber, while the buffer tube experiences contraction or elongation. If the buffer tube contracts, due to cold temperatures, for example, then the buffer tube pitch and diameter tend to decrease. As the fiber tends to keep the same length, it would then attempt to increase its helix path diameter; that is, to migrate radially outward within the buffer tube until the inner surface of the buffer tube is reached. If the buffer tube elongates, due to tension on the cable, for example, the fiber tends to migrate inward within the buffer tube until the inner wall of the buffer tube nearest the cable center is reached to avoid tension. When these extreme points are reached, further contraction or elongation will rapidly increase bends in the glass fiber, resulting in increased attenuation and cable failure. This is due to the fact that the fiber no longer has room to move.
These considerations impact on cable design. Within a buffer tube of a given diameter, placing more optical fibers within a buffer tube may be more cost-effective, but the ability of the cable to withstand temperature and physical stress decreases. Similarly, ability to withstand these stresses increases as buffer tube size increases, but at the price of making the cable larger.